The present invention relates to a load cell having strain gauge resistors and conductive layers formed by a thin film technique.
Heretofore, a load cell having strain gauge resistors formed through insulation layers on a beam body having a strain generating section and a conductive pattern for forming a bridge circuit by coupling these strain gauge resistors is known. In the load cell of this type, these strain gauge resistors and the conductive pattern are formed, for example, by vapor deposition, sputtering, or plating. In this case, in order to reduce the resistance of the conductive pattern, the conductive pattern is formed thick. Thus, when the conductive pattern is formed to have a strain therein, a stress is produced by this internal strain in the conductive pattern. This stress varies with age and is transmitted, for example, through an insulation layer to the strain guage resistors. As a result, the resistance values of the strain gauge resistors change. Correspondingly, bridge balance or span (the output voltage of the load cell) resultantly vary with age, thereby causing the performance of the load cell to be deteriorated.
In a load cell of this type, the conductive pattern is composed of a conductive layer selectively formed on a resistive layer for forming strain gauge resistors. Thus, when this load cell is used at different temperatures, the resistive layer under this conductive layer is contracted or expanded in response to the contraction or expansion of the conductive layer due to the difference of thermal expansion between the resistive layer and the conductive layer. A stress generated by this contraction or expansion is transmitted through the insulation layer to the strain gauge resistors, thereby altering the resistance value of the strain gauge resistors to cause the bridge balance or span to be varied.